I am trying to get these damn dimensions perfect before I do any cutting and, well, I am going ---smurfing--- crazy!

I have all the dimensions but what I really need are all the angles to cut the pieces at in order for them to sit flush against each other.
Could anyone help out in anyway? I ma not even sure if there is a program (maybe autocad) that would tell you what size the pieces need to be, at what angle you have to cut them at including the extra length needed if you are cutting an angle "outside" the width it states.
For example: I have a piece that is 250mm in length, 12mm thick and I need a 45 deg cut facing OUTWARDS on each side. Geometry would tell me that I need a piece that is 274mm in length and then trim that inwards 45 degs in order to get the 250mm length that I need. Does that make any sense?

Franco has helped me out constantly and to be honest I feel a bit bad asking him again because he is trying to finish his juke.
If anyone is able to use sketch-up to draw up a model I would also appreciate it SO MUCH before I actually jump out of the building I am in!!!
I can provide the measurements and directions.
Thanks guys!